Highly selective oxide etch process using hexafluorobutadiene

ABSTRACT

An oxide etching process, particular useful for selectively etching oxide over a feature having a non-oxide composition, such as silicon nitride and especially when that feature has a corner that is prone to faceting during the oxide etch. The invention preferably uses the unsaturated 4-carbon fluorocarbons, specifically hexafluorobutadiene (C 4 F 6 ), which has a below 10°C. and is commercially available. The hexafluorobutadiene together with argon is excited into a high-density plasma in a reactor which inductively couples plasma source power into the chamber and RF biases the pedestal electrode supporting the wafer. Preferably, a two-step etch is used process is used in which the above etching gas is used in the main step to provide a good vertical profile and a more strongly polymerizing fluorocarbon such as difluoromethane (CH 2 F 2 ) is added in the over etch to protect the nitride corner.

RELATED APPLICATION

This application is a division of Ser. No. 09/193,056, filed Nov. 16,1998, now issued as U.S. Pat. No. 6,174,451, which is a continuation inpart of Ser. No. 09/049,862, filed Mar. 27, 1998, now issued as U.S.Pat. No. 6,183,655.

FIELD OF THE INVENTION

The invention relates generally to etching of silicon integratedcircuits. In particular, the invention relates to etching silicon oxideand related materials in a process that is capable of greatly reducedetching rates for silicon nitride and other non-oxide materials butstill producing a vertical profile in the oxide.

BACKGROUND ART

In the fabrication of silicon integrated circuits, the continuingincrease in the number of devices on a chip and the accompanyingdecrease in the minimum feature sizes have placed increasingly difficultdemands upon many of the many fabrication steps used in theirfabrication including depositing layers of different materials ontosometimes difficult topologies and etching further features within thoselayers.

Oxide etching has presented some of the most difficult challenges. Oxideis a somewhat generic term used for silica, particularly silicon dioxide(SiO₂) although slightly non-stoichiormetric compositions SiO_(x) arealso included, as is well known. The term oxide also covers closelyrelated materials, such as oxide glasses including borophosphosilicateglass (BPSG). Some forms of silicon oxynitride are considered to moreclosely resemble a nitride than an oxide. Small fractions of dopantssuch as fluorine or carbon may be added to the silica to reduce itsdielectric constant. Oxide materials are principally used forelectrically insulating layers, often between different levels of theintegrated circuit. Because of the limits set by dielectric breakdown,the thickness of the oxide layers cannot be reduced to much below 0.5 to1 μm. However, the minimum feature sizes of contact and via holespenetrating the oxide layer are being pushed to well below 0.5 μm. Theresult is that the holes etched in the oxide must be highly anisotropicand must have high aspect ratios, defined as the depth to the minimumwidth of a hole. A further problem arises from the fact that theunderlying silicon may be formed with active doped regions ofthicknesses substantially less than the depth of the etched hole (theoxide thickness). Due to manufacturing variables, it has becomeimpossible to precisely time a non-selective oxide etch to completelyetch through the silicon oxide without a substantial probability of alsoetching through the underlying active silicon region.

The anisotropy can be achieved in dry plasma etching in which an etchinggas, usually a fluorocarbon, is electrically excited into a plasma. Theplasma conditions may be adjusted to produce highly anisotropic etchingin many materials. However, the anisotropy should not be achieved byoperating the plasma reactor in a pure sputtering mode in which theplasma ejects particles toward the wafer with sufficiently high energythat they sputter the oxide. Sputtering is generally non-selective, andhigh-energy sputtering also seriously degrades semiconducting siliconexposed at the bottom of the etched contact hole.

In view of these and other problems, selective etching processes havebeen developed which depend more upon chemical effects. These processesare often described as reactive ion etching (RIE). A sufficiently highdegree of selectivity allows new structures to be fabricated without theneed for precise lithography for each level.

An example of such an advanced structure is a self-aligned contact(SAC), illustrated in the cross-sectional view of FIG. 1. A SACstructure for two transistors is formed on a silicon substrate 2. Apolysilicon gate layer 4, a tungsten silicide barrier and glue layer 6,and a silicon nitride cap layer 8 are deposited andphotolithographically formed into two closely spaced gate structures 10having a gap 12 therebetween. Chemical vapor deposition is then used todeposit onto the wafer a substantially conformal layer 14 of siliconnitride (Si₃N₄), which coats the top and sides of the gate structures 10as well as the bottom 15 of the gap 12. In practice, the nitridedeviates from the indicated stoichiometry and may have a composition ofSiN_(x), where x is between 1 and 1.5. The nitride acts as an electricalinsulator. Dopant ions are ion implanted using the gate structures 10 asa mask to form a self-aligned p-type or n-type well 16, which acts as acommon source for the two transistors having respective gates 10. Thedrain structures of the transistors are not illustrated.

An oxide field layer 18 is deposited over this previously definedstructure, and a photoresist layer 20 is deposited over the oxide layer18 and photographically defined into a mask so that a subsequent oxideetching step etches a contact hole 22 through the oxide layer 18 andstops on the portion 24 of the nitride layer 14 underlying the hole 22.It is called a contact hole because the metal subsequently depositedinto the contact hole 22 contacts silicon rather than a metal layer. Apost-etch sputter removes the nitride portion 24 at the bottom 15 of thegap 12. The silicon nitride acts as an electrical insulator for themetal, usually aluminum, thereafter filled into the contact hole 22.

Because the nitride acts as an insulator, the SAC structure and processoffer the advantage that the contact hole 22 may be wider than the widthof the gap 12 between the gate structures 10. Additionally, thephotolithographic registry of the contact hole 22 with the gatestructures 10 need not be precise. However, to achieve these beneficialeffects, the SAC oxide etch must be highly selective to nitride. Thatis, the process must produce an oxide etch rate that is much greaterthan the nitride etch rate. Numerical values of selectivity arecalculated as the ratio of the oxide to nitride etch rates. Selectivityis especially critical at the corners 26 of the nitride layer 14 aboveand next to the gap 12 since the corners 26 are the portion of thenitride exposed the longest to the oxide etch. Furthermore, they have ageometry favorable to fast etching that tends to create facets at thecomers 26.

Furthermore, increased selectivity is being required with the increasedusage of chemical mechanical polishing (CMP) for planarization of anoxide layer over a curly wafer. The planarization produces a flat oxidesurface over a wavy underlayer substrate, thereby producing an oxidelayer of significantly varying thickness. As a result, the time of theoxide etch must be set significantly higher, say by 100%, than the etchof the design thickness to assure penetration of the oxide. This iscalled over etch, which also accounts for other process variations.However, for the regions with a thinner oxide, the nitride is exposedthat much longer to the etching environment.

Ultimately, the required degree of selectivity is reflected in theprobability of an electrical short between the gate structures 10 andthe metal filled into the contact hole 22. The etch must also beselective to photoresist, for example at facets 28 that develop at themask comers, but the requirement of photoresist selectivity is not sostringent since the photoresist layer 20 may be made much thicker thanthe nitride layer 14.

In the future, the most demanding etching steps are projected to beperformed with high-density plasma (HDP) etch reactors. Such HDP etchreactors achieve a high-density plasma having a minimum average iondensity of 10¹¹cm⁻³ across the plasma exclusive of the plasma sheath.Although several techniques are available for achieving a high-densityplasma such as electron cyclotron resonance and remote plasma sources,the commercially most important technique involves inductively couplingRF energy into the source region. The inductive coil may becylindrically wrapped around the sides of chamber or be a flat coilabove the top of the chamber or represent some intermediate geometry.

An example of an inductively coupled plasma etch reactor is the IPS etchreactor, which is also available from Applied Materials and which byCollins et al. describe in U.S. patent application, Ser. No. 08/733,544,filed Oct. 21, 1996 and in European Patent Publication EP-840,365-A2. Asshown in FIG. 2, a wafer 30 to be processed is supported on a cathodepedestal 32 supplied with RF power from a first RF power supply 34. Asilicon ring 36 surrounds the pedestal 32 and is controllably heated byan array of heater lamps 38. A 29 grounded silicon wall 40 surrounds theplasma processing area. A silicon roof 42 overlies the plasma processingarea, and lamps 44 and water cooling channels 46 control itstemperature. The temperature-controlled silicon ring 36 and silicon roof42 may be used to scavenge fluorine from the fluorocarbon plasma. Forsome applications, fluorine scavenging can be accomplished by a solidcarbon body, such as amorphous or graphitic carbon, or by othernon-oxide silicon-based or carbon-based materials, such as siliconcarbide.

Processing gas is supplied from one or more bottom gas feeds 48 througha bank of mass flow controllers 50 under the control of a systemcontroller 52, in which is stored the process recipe in magnetic orsemiconductor memory. Gas is supplied from respective gas sources 54,56, 58. The conventional oxide etch recipe uses a combination of afluorocarbon or hydrofluorocarbon and argon. Octafluorocyclobutane(C₄F₈) and trifluoromethane (CHF₃) are popular fluorocarbons, but otherfluorocarbons, hydrofluorocarbons, and combinations thereof are used.

An unillustrated vacuum pumping system connected to a pumping channel 60around the lower portion of the chamber maintains the chamber at apreselected pressure.

In the used configuration, the silicon roof 42 is grounded, but itssemiconductor resistivity and thickness are chosen to pass generallyaxial RF magnetic fields produced by an inner inductive coil stack 62and an outer inductive coil stack 64 powered by respective RF powersupplies 66, 68.

Optical emission spectroscopy (OES) is a conventional monitoring processused for end-point detection in plasma etching. An optical fiber 70 isplaced in a hole 72 penetrating the chamber wall 40 to laterally viewthe plasma area 74 above the wafer 30. An optical detector system 76 isconnected to the other end of the fiber 70 and includes one or moreoptical filters and processing circuitry that are tuned to the plasmaemission spectrum associated with the aluminum or copper species in theplasma. Either the raw detected signals or a trigger signal iselectronically supplied to the controller 52, which can use the signalsto determine that one step of the etch process has been completed aseither a new signal appears or an old one decreases. With thisdetermination, the controller 52 can adjust the process recipe or endthe etching step.

The IPS chamber can be operated to produce a high-density or alow-density plasma. The temperature of the silicon surfaces and of thewafer can be controlled. The bias power applied to the cathode 42 can beadjusted independently of the source power applied to the coils 62, 64.

It has become recognized, particularly in the use of HDP etch reactors,that selectivity in an oxide etch can be achieved by a fluorocarbonetching gas forming a polymer layer upon the non-oxide portions, therebyprotecting them from etching, while the oxide portions remain exposed tothe etching environment. It is believed that the temperature controlledsilicon ring 36 and roof 42 in the reactor of FIG. 2 control thefluorine content of the polymer, and hence its effectiveness againstetching by the fluorocarbon plasma, when the polymer overlies anon-oxide. However, this mechanism seems to be responsible for at leasttwo problems if high selectivity is being sought. If excessive amountsof polymer are deposited on the oxide or nitride surfaces in the contacthole being etched, the hole can close up and the etching is stoppedprior to complete etching of the hole. This deleterious condition isreferred to as etch stop.

Further, the chemistry may be such that the polymer formation dependscritically upon the processing conditions. It may be possible to achievehigh selectivity with one set of processing conditions, but very smallvariations in those parameters may be enough to substantially reduce theselectivity on one hand or to produce etch stop on the other. Suchvariations can occur in at least two ways. The conditions at the middleof the wafer may vary from those at the center. Furthermore, theconditions may change over time on the order of minutes as the chamberwarms up or on the order of days as the equipment ages or as chamberparts are replaced. It is felt that hardware can be controlled to nobetter than ±5 or 6%, and a safety margin or 3 to 6 is desired. Massflow controllers 46 are difficult to control to less than ±1 sccm(standard cubic centimeter per minute) of gas flow so gas flows of anyconstituent gas of only a few sccm are prone to large percentagevariations.

These factors indicate that a commercially viable etch process must havea wide process window. That is, moderate variations in such parametersas gas composition and chamber pressure should produce only minimalchanges in the resultant etching.

Several oxide etch processes have been proposed which rely uponhigher-order hydrogen-free fluorocarbons and hydrofluorocarbons, bothgenerically referred to as fluorocarbons. Examples of higher-orderfluorocarbons are fluoroethane, fluoropropane, and even fluorobutane,both in its linear and cyclic forms. In U.S. Pat. No. 5,423,945, Markset al. disclose an oxide etch selective to nitride using C₂F₆ in an HDPetch reactor having a thermally controlled silicon surface. Laterprocess work with the IPS chamber of FIG. 2 has emphasized C₄F₈ as theprincipal etchant species. Wang et al. have disclosed the use offluoropropanes and fluoropropylenes, e.g., C₃F₆ and C₃H₂F₆, in U.S.patent applications, Ser. Nos. 08/964,504 and 09/049,862, filed Nov. 5,1997 and Mar. 27, 1998 respectively. The two examples of fluorocarbonshave F/C ratios of 2, as does C₄F₈, and some researchers, includingYanagida in U.S. Pat. No. 5,338,399, believe this value produces thebest passivating polymer. We have observed, however, that the etchingprofile cannot be controlled with C₃H₂F₆.

If possible, it is desirable to use the already widely availablefluoromethanes, which include carbon tetrafluoride (CF₄),trifluoromethane (CHF₃), difluoromethane (CH₂F₂), and monofluoromethane(CH₃F). Hung et al. in U.S. patent application, Ser. No. 08/956,641,filed Oct. 23, 1997, suggest the use of CHF₃ and CH₂F₂. We have observedthat this last combination is insufficiently selective, indicating poorpolymer formation.

Although octafluorocyclobutane (C₄F₈) remains the most popular oxideetching gas, we observe that it suffers from too narrow a processwindow. Furthermore, although C₄F₈ is known to provide selectivity atthe bottom of the etching hole, it provides little sidewall passivation,which is required for the desired vertical profiles. Also, C₄F₈ has aboiling point of +1° C., which is considered somewhat high, especiallyin view of a trend to operate with very cold cathodes. Often carbonmonoxide (CO) is added to C₄F₈ to increase the sidewall passivation aswell as increase general nitride selectivity. However, CO is not onlytoxic, it also forms carbonyls with nickel and iron in gas cylinders andsupply lines. The carbonyls are believed to contaminate wafers. Forthese reasons, the use of CO is not preferred.

The two approaches using alternatively the fluoromethanes andhexafluoropropane (C₃H₂F₆) both provide wider process windows withsatisfactory etching characteristics, but we still believe that theprocess windows are too narrow and the etching characteristics can befurther improved.

Hexafluoropropylene (C₃F₆) has also been investigated by Wang et al. inthe previously cited patents. It has the F/C ratio desired by Yanagida.However, the results show insufficient selectivity.

A theoretically promising etching gas is tetrafluoroethylene (C₂F₄).However, it is considered dangerously explosive.

There are further considerations in selecting fluorocarbons for oxideetching. If a higher-order fluorocarbon is selected, a presentlyavailable commercial supply is greatly desired, even if a semiconductorgrade needs to be developed. Furthermore, many of the higher-orderfluorocarbons are liquids at near to room temperature. It is stillpossible to use liquid precursors by the use of bubblers to atomize theliquid in a carrier gas. However, bubblers present another expense, theyneed frequent maintenance, and the effective flow rate of the liquidprecursor is difficult to tightly control. Gaseous precursors are muchmore preferred.

For these reasons, other fluorocarbon etching gases are desired.

SUMMARY OF THE INVENTION

The invention may be summarized as an oxide etching process usingunsaturated higher-order fluorocarbons with a low hydrogen content. Theprimary examples are hexafliiorobutadiene (C₄F₆), pentafluoropropylene(C₃HF₅), and trifluoropropyne (C₃HF₃), most preferablyhexafluorobutadiene. When nitride facet selectivity is required, a moreheavily polymerizing gas should be added, such as hydrofluoromethane,preferably difluoromethane (CH₂F₂). A neutral working gas such as argon(Ar) is preferably used in a high-density plasma reactor. A wide processwindow is achieved when the pressure is kept below 20 milliTorr and thebias power nearly equals the source power. The etching is preferablydivided into two substeps. The first substep includes little or noheavily polymerizing gas and is tuned for vertical profile in the oxide.The second substep includes the heavily polymerizing gas and is tunedfor nitride selectivity and no etch stop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a self-aligned contact (SAC)structure.

FIG. 2 is a schematic view, partly in cross section, of an inductivelycoupled high-plasma density etch reactor.

FIGS. 3 through 5 illustrate chemical structures of three etching gasesof the invention.

FIG. 6 is a flow diagram of one embodiment of the integrated etchingprocess of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We believe that higher-order fluorocarbons, specifically those havingthree carbons or more, and those having a F/C ratio of one or preferablygreater but less than two will produce a polymer providing satisfactoryetching characteristics with a wide process window. The fluorocarbons ofmore than four carbons are unnecessarily complex, especially since theytend to break up in a high-density plasma. A high-density plasma tendsto dissociate gas molecules into radicals. A longer carbon chainprovides a larger variety of activated particles, including the CF₂*radical usually believed to be responsible for the formation of thefluorocarbon polymer chain. The longer carbon precursor atoms provide alarger variety of particles, which may be helpful in cross linking thepolymer and the distribution of which may be controlled by differentpower levels in the HDP reactor. It is known that a low-fluorine polymerprovides better selectivity probably because of increased cross linkingresulting in a tougher, more protective polymer. Although an F/C ratioof two corresponds to a single segment of a fluorocarbon chain, it doesnot account for polymeric cross linking. Silicon-based fluorinescavenging is an attempt at reducing the fluorine content by scavengingany free radicals in the plasma. The longer carbon molecules with F/Cratios of less than two provide another mechanism for reducing thefluorine content of the polymer.

The fluoropropane gases C₃HF₇ and C₃H₂F₆ chosen by Wang et al. in theabove cited patents were intended to satisfy at least some of theserequirements. However, as mentioned above, at least C₃H₂F₆ produces poorprofiles. However, there is no reason to restrict the 3-carbonfluorocarbon to fluoropropanes. Unsaturated 3-carbon fluorocarbons canalso be used. Three commercially available unsaturated fluorocarbon andhydrodrofluorocarbon gases satisfy these requirements. An unsaturatedfluorocarbon is one having a double or a triple bond between neighboringcarbons.

A first example of such a gas is hexafluorobutadiene (C₄F₆). The isomerhexafluoro-1,3-butadiene has a chemical structure illustrated in FIG. 3with four carbon atoms, six fluorine atoms, and two double bonds. ItsF/C ratio is 1.5. It has a boiling point of 6-7° C.

A second example is trifluoropropyne (C₃HF₃ and alternatively namedtrifluoromethylacetylene). The isomer 3,3,3-trifluoro-1-propyne has achemical structure illustrated in FIG. 4 with three carbon atoms, threefluorine atoms, one hydrogen atom, and one triple bond. Its F/C ratiois 1. It has a boiling point of −48° C.

A third example is pentafluoropropylene (C₃HF₅ and alternatively namedpentafluoropropene). The isomer 1,1,3,3,3-pentafluoropropylene has achemical structure illustrated in FIG. 5 with three carbon atoms, fivefluorine atoms, one hydrogen atom, and one double bond. Its F/C ratio is1.67. It has a boiling point of −21° C. Wang et al. in the parent U.S.patent application Ser. No. 09/049,862 suggests C₃HF₅ as a substitutefor C₃F₆.

Other isomers may be available with only slightly changed boilingpoints. No attempt has been made to characterize the stereoisomers. Allthe exemplary fluorocarbons form in linear chains.

Experimental data was obtained for the use of hexafluorobutadiene (C₄F₆)as the primary etching gas. This gas was chosen for the experimentsbecause of its ready commercial availability. Fukuta has previouslydisclosed an oxide etching process using C₄F₆ in Laid-Open JapanesePublished Application (Kokai) 9-191002. However, he uses a magneticallyenhanced capacitively coupled etch chamber. His structure requiresselectivity only at the bottom of a via hole, and he does not addressthe high selectivity required for via holes in advanced processes likeSAC. The capacitively coupled reactor does not allow the decoupling ofthe source and bias power.

A first etch recipe is listed in TABLE 1.

TABLE 1 First Second Etch Etch Recipe Recipe C₄F₆ Flow (sccm) 20 20CH₂F₂ Flow (sccm) 0 15 Ar Flow (sccm) 100 100 Inner Source Power (W) 180180 Outer Source Power (W) 1260 1260 Bias Power (W) 1400 1400 Pressure(mTorr) 4 4 Roof Temp. (° C.) 200 200 Ring Temp. (° C.) 270 270 CathodeTemp. (° C.) +10 +10 Backside He Pressure (T) 7 7 Time (s) 120 120

This recipe was tested in the IPS chamber illustrated in FIG. 2. Therecipe is characterized as having a high argon content, a much higherouter coil power than inner coil power by at least a factor of 5 so asto concentrate the magnetic field at the outer periphery, a lowpressure, and a relatively high silicon ring temperature. The bias poweris relatively high compared to the source power of between 50% and 150%.However, the separate control of the bias power is considered importantfor obtaining high nitride selectivity with reasonable etching rates.The source power controls the ion and radical flux while the bias powercontrols the ion energy incident on the wafer. Too high an ion energywill resemble non-selective sputtering. The high-density plasma isfurther important because it produces a higher fraction of ionizedetching particles, which can be directed to the bottom of holes withhigh aspect ratios.

Two SAC structures, as illustrated in FIG. 1, were used in theexperimental work. A first, short SAC structure has a TEOS oxidethickness of about 0.55 μm on top of a nitride-covered gate structure 10having a height of about 0.3 μm. That is, the total oxide etch depth is0.85 μm.

The first recipe produces an oxide etch rate of about 600 nm/min. Theprofile varies over the wafer with angular values of about 87°. However,the selectivity is about 20:1 at the corner, and severe faceting isobserved at the nitride corner.

No etch stop is observed with this recipe or with any of the otherreported recipes, and it will not be hereafter referenced.

A recipe similar to the first C₄F₆ recipe was compared to an optimizedrecipe using hexafluoropropane (C₃H₂F₆) that is somewhat similar to theC₄F₆ recipe. The recipes were tested on a second, tall SAC structurehaving 0.75 μm of TEOS oxide over a 0.45 μm-high gate structure for atotal oxide etch depth of 1.2 μm and with a trench opening of about 0.35μm. The results for C₄F₆ are about the same as described above. TheC₃H₂F₆ etch produces a V-shaped profile with a sidewall angle of 83-85°.The same profile is observed in the nitride, indicating no effectiveselectivity. The same recipes were applied to a simulated structurehaving 1.2 μm of oxide over nitride but no gate structure. The C₄F₆recipe shows some inward flaring below about 0.75 μm, but this shouldnot present a problem if both sidewalls fall on respective gatestructures and nitride selectivity is adequately high. The C₃H₂F₆ recipeproduces the same V-shaped profile to the bottom of the 1.2 μm-thickoxide.

In order to increase the nitride selectivity for the C₄F₆ recipe and todecrease the nitride faceting, a more heavily polymerizing fluorocarbongas may be added to the etching mixture. A fluoromethane is preferred.Difluorofluoromethane (CH₂F₂) is less polymerizing thanmonofluoromethane (CH₃F) so that standard mass flow controllers canadequately meter its flow. A second etch recipe listed in TABLE 1 wasused having somewhat less CH₂F₂ than C₄F₆. The second recipe was usedwith the first, shallow SAC structure described above. The observedoxide etch rate is about 620 nm/min, and the nitride selectivity issignificantly improved to about 30:1. However, the profile angle whenthe entire etch uses the second recipe is substantially degraded toabout 84°.

To combine the best features of both etch recipes, a two-step oxide etchrecipe has been developed. As illustrated in the process flow diagram ofFIG. 6, a main etch 80 is first performed using an etching gas mixtureof C₄F₆ and argon. The main etch provides a fast etch rate and goodvertical profiles. Then, an over etch 82 is performed in which thepolymerizing CH₂F₂ is added in order to increase the nitride selectivityand to thus protect the nitride corners. Vertical profile is not soimportant in the over etch, especially when the main etch has alreadyreached the nitride.

A first embodiment of the two-step etch recipe is summarized in TABLE 2.

TABLE 2 Main Over Etch Etch C₄F₆ Flow (sccm) 20 20 CH₂F₂ Flow (sccm) 010 Ar Flow (sccm) 100 100 Inner Source Power (W) 0 0 Outer Source Power(W) 1600 1600 Bias Power (W) 1400 1400 Pressure (mTorr) 10 10 Roof Temp.(° C.) 230 230 Ring Temp. (° C.) 300 300 Cathode Temp. (° C.) +15 +15Backside He Pressure (T) 7 7 Time (s) 100 30

This recipe was applied to the second, tall SAC structure. The profileangle in the oxide portion of the hole is observed to be at least 87°,and the nitride selectivity is acceptable.

The timing of the switch between the main etch and the over etch ischosen such that the main etch reaches the nitride bottom at most of thelocations on the wafers and the over etch guarantees a complete etchwithout producing excessive nitride faceting. To provide further nitrideprotection, the changeover can be moved to a point where the main etchhas passed the top of the nitride at most locations, that is, issomewhere in the gap between the gate structures. The changeover can becontrolled dynamically by relying on optical emission spectroscopy orequivalent means tuned to an emission of a nitride byproduct. Thereby,the polymerizing gas is added as soon as the main etch has reached thetop nitride at a significant number of locations.

A number of experiments were then performed to determine the processwindow and the fact that it is relatively wide. In a first pair ofexperiments, a one-step C₄F₆/CH₂F₂ etch was performed on the shallow SACstructure having a 0.35 μm trench aperture following alternatively thetwo variation recipes listed in TABLE 3.

TABLE 3 First Second Var. Var. C₄F₆ Flow (sccm) 17 25 CH₂F₂ Flow (sccm)5 5 Ar Flow (sccm) 100 100 Inner Source Power (W) 0 0 Outer Source Power(W) 1600 1600 Bias Power (W) 1400 1400 Pressure (mTorr) 4.5 4.5 RoofTemp. (° C.) 230 230 Ring Temp. (° C.) 300 300 Cathode Temp. (° C.) +10+10 Backside He Pressure (T) 7 7 Time (s) 150 150

These two recipes vary the flow of C₄F₆ by 15% about its baseline valueof 20 sccm. The other parameters are somewhat different from thebaseline values of TABLE 2, but are close enough to establish a windowfor the C₄F₆ in the critical over etch. Nitride corner etching is barelyobservable. The minimum profile angle varied between 3° and 4° betweenthe two recipes, which is better than the 84° of the second etch recipeof TABLE 1. Inward tapering occurs at the bottom of the etch. Similarresults are observed with 0.4 μm trenches.

This ±15% window for C₄F₆ flow is to be compared with the window for anoptimized C₄F₈ recipe of 15 sccm of C₄F₆ and 28 sccm of CH₂F₂ with otherparameters similar to those for a C₄F₆ etch. A 20% reduction of the C₄F₈flow to 12 sccm produces severe nitride faceting to the extent of punchthrough at some locations. A 20% increase to 18 sccm produces very goodnitride corner selectivity but increases tapering to the extent thatsome oxide is not etched at the side of the gate structure.

The baseline recipe of TABLE 2 was varied to decrease the silicon ringtemperature to 255° C. in both the main etch and the over etch. For boththe shallow and tall SAC structures, the lower ring temperatureincreases the profile angle to at least 88° and eliminates sidewalloxide that tends to form on the walls of the nitride at the bottom ofthe trench. Nitride corner selectivity decreases somewhat, but it isstill acceptable.

In two further experiments, the over etch recipe was modified to include20 sccm of C₄F₆ and 5 sccm of CH₂F₂. A satisfactory etch is obtained atsilicon temperatures of 300° C. for the ring and 230° C. for the roof.However, if the ring temperature is reduced to 245° C. and the rooftemperature to 220° C. with the same flow of CH₂F₂, the nitride corneris severely faceted. Nonetheless, the silicon scavenging by the hotsilicon parts can be traded off against the polymerization produced bythe CH₂F₂. Third and fourth variant recipes for the over etch are listedin TABLE 4.

TABLE 4 Third Fourth Var. Var. C₄F₆ Flow (sccm) 20 20 CH₂F₂ Flow (sccm)15 5 Ar Flow (sccm) 100 100 Inner Source Power (W) 0 0 Outer SourcePower (W) 1600 1600 Bias Power (W) 1400 1400 Pressure (mTorr) 4.5 4.5Roof Temp. (° C.) 200 230 Ring Temp. (° C.) 245 300 Cathode Temp. (° C.)+10 +10 Backside He Pressure (T) 7 7 Time (s) 150 150

Both variant recipes applied to a one-step etch of the shallow SACstructure show a satisfactory etch. Nitride corner etching is notreadily observable, and the profile angle is at least 86°, which isacceptable for the over etch.

Two sets of experiments were performed to determine the sensitivity topressure variations. In the first set, a main etch recipe with no CH₂F₂was used to etch the entire oxide layer, both in a shallow and in a tallSAC structure. The pressure was alternatively set at 10, 15, and 20milliTorr. The minimum profile angles decreases from 88° at 10milliTorr, to 87° at 15 milliTorr, and to 86° at 20 milliTorr, allconsidered to be acceptable values. The previously described lowersidewall tapering is observed at all pressures. Without the polymerizingCH₂F₂, significant but not severe nitride corner faceting is observed,particularly at the lower pressure, and a 100s etch tends to punchthrough the bottom nitride layer. However, the over etch recipe isintended to circumvent the poor nitride selectivity of the main etchrecipe. Thus, over the pressure range of 10 to 20 milliTorr, the mainetch performs satisfactorily.

In a second set of experiments, an over etch recipe was used to etch ashort SAC structure and the pressure was alternately set to 4.5 and 7milliTorr. No nitride corner etching is readily observable at eitherpressure.

Similar results are expected with the other two unsaturatedfluorocarbons, trifluoropropyne (C₃HF₃) and pentafluoropropylene(C₃HF₅).

In the integrated etching process of FIG. 6, after the completion of themain etch 80 and the over etch 82, an ashing step 84 removes theremaining photoresist and deposited polymer, usually with an unbiasedoxygen plasma. Then, a nitride etch step 86 removes the nitride portion26 remaining at the bottom of the hole 22, as illustrated in FIG. 1.This step typically uses a fluorocarbon, such as CH₂F₂, in combinationwith argon and oxygen in a soft plasma etch. The oxygen destroys anynitride selectivity. Hung et al. describe these final steps in U.S.patent application Ser. No. 09/149,810, filed Sep. 8, 1998.

Although the main etching steps described above did not use anypolymerizing fluoromethane, it is understood that nearly the same effectcould be achieved by flowing in the main etch no more than 20% of thefluoromethane used in the over etch.

Although argon is the usual chemically inactive carrier gas, other gasescan be substituted, such as the other rare gases, such as neon.

Although the description above concentrated on selectivity to nitride,the mechanisms involved in selective oxide etching depend principally asfar as materials are concerned upon whether the layer contains asignificant amount of oxygen or not. Therefore, the same chemistry isapplicable to etching oxide over a non-oxide layer or feature.

The examples reported above were obtained on the inductively coupled IPSreactor capable of producing a high-density plasma. Other inductivelycoupled plasma etch reactors are available with a variety of coilconfigurations. The current inductively coupled HDP reactors have theadvantage of decoupling the source power from the bias power, thusallowing a reasonable etching rate with low ion energies. There areother ways of decoupling the source and bias power, for example, with aremote plasma source (RPS) or with an electron-cyclotron resonance (ECR)reactor.

Although the above results were derived from experiments performed onSAC structures, there are other structures in which high nitride cornerselectivity is required. A pair of examples include structures in whicha nitride layer is used as a mask. A first example is a silicon nitridehard mask placed between the photoresist and the oxide layer. A firstetching step etches the photoresist pattern into the hard mask.Thereafter, a second etching step etches the oxide layer according tothe pattern of the hard mask. In the second step, photoresistselectivity is not required, but the nitride corner of the hard mask isexposed throughout most of the second step so that high nitride cornerselectivity is required. A second example is a self-aligned localinterconnect (SALI), as Wang et al. describe in U.S. patent applicationSer. No. 08/964,504, filed Nov. 5, 1998. In the SALI structure thenitride covered gate structure may be completely exposed, and further aplanar bottom nitride is exposed for a long period. A third example is adual-damascene structure, as Tang et al. describe in U.S. patentapplication Ser. No. 09/112,864, filed Jul. 9, 1998. In this structure,a nitride layer separates two levels of oxide. Because nitride acts as astop layer, one etching step, sometimes in combination by an initialetch, can etch the lower oxide layer into one feature and concurrentlyetch the upper oxide layer into a connected, larger feature. Thepatterned nitride layer operates either as a stop layer or as anintermediate mask. The intermediate nitride layer is substantiallyplanar but has a corner surrounding an aperture opening into the lowerdielectric layer.

The oxide etching process of the invention thus provides superioretching characteristics with the use of gases which are novel tosemiconductor processing but which are commercially available. The otherprocess parameters are achievable in commercially available plasmareactors.

What is claimed is:
 1. An oxide etching process for etching an oxidelayer in a substrate having a silicon nitride layer exposed during saidprocess, comprising the steps of: flowing into a plasma reaction chamberan etching gas mixture comprising hexafluorbutadiene, ahydrofluoromethane, and a chemically inactive carrier gas; and excitingsaid etching gas mixture into a plasma to etch said oxide layer withselectivity to said silicon nitride layer.
 2. The process of claim 1,wherein said exciting step couples RF energy into a plasma source regionover a pedestal electrode supporting said substrate and furthercomprising RF biasing said pedestal electrode.
 3. The process of claim1, wherein said silicon nitride layer comprises a hard mask overlyingsaid oxide layer.
 4. An oxide etching process for etching an oxide layerin a substrate having a non-oxide layer exposed during said process,comprising the steps of: flowing into a plasma reaction chamber anetching gas mixture comprising hexafluorobutadiene, difluoromethane, anda chemically inactive carrier gas; and exciting said etching gas mixtureinto a plasma to etch said oxide layer with selectivity to saidnon-oxide layer.
 5. An oxide etch process for etching an oxide layer ina substrate having a non-oxide layer exposed during said process, saidprocess comprising the steps of: supporting said substrate on a pedestalelectrode within a plasma reaction chamber; flowing into said plasmareaction chamber an etching gas mixture comprising hexafluoroburadieneand a chemically inactive carrier gas; RF biasing said pedestalelectrode; and preferentially coupling RF energy into said chamber sothat energy coupling in a peripheral region of said chamber isrelatively higher with respect to energy coupling in a central region ofsaid chamber.
 6. The process of claim 5, wherein said chamber comprisestwo or more inductive coils including an inner coil and a surroundingouter coil both disposed in back of a roof of said chamber facing saidpedestal electrode, wherein more RF energy is coupled into said outercoil than into said inner coil.
 7. The process of claim 6, wherein moreRF energy is coupled into said outer coil than into said inner coil byat least a factor of five.
 8. An etching process for etching an oxidelayer selectively to a non-oxide layer exposed daring an etching of saidoxide layer, comprising: a first step of flowing into a plasma reactionchamber a first etching gas mixture comprising hexafluorobutadiene, anda chemically inactive carrier gas and exciting said first etching gasmixture into a plasma; and a subsequent second step of flowing into saidplasma reaction chamber a second etching gas mixture comprisinghexafluorobutadiene, said chemically inactive gas, and a more heavilypolymerizing fluorocarbon gas than said hexafluorobutadiene and excitingsaid second etching gas mixture into a plasma to etch said oxide layerselectively to said non-oxide layer, wherein an amount of said moreheavily polyrmerizing gas in said second step is greater than in saidfirst step.
 9. The process of claim 8, wherein said non-oxide layercomprises a nitride layer.
 10. The process of claim 8, wherein said moreheavily polymerizing fluorocarbon gas comprises a hydrofluoromethane.11. The process of claim 10, wherein said hydrofluoromethane comprisesdifluoromethane.
 12. The process of claim 8, wherein said oxide layeroverlies said non-oxide layer.
 13. The process of claim 8, wherein saidnon-oxide layer is a nitride layer overlying said oxide layer.